Hello,
I do not know of any National Standard (per se) that includes this process. This is generally something that we do per a specific Company's procedure (if that exists).
I like to use a "water level" (comprising a 100 foot length of clear plastic tubing which contains water) to do the job. I also have a set of 8 laser equiped "torpedo levels" that are useful. It will usually take 2 or 3 people working as a team.
I always check the slope from hanger-to-hanger and (when I can get to them) I like to include the length from hangers to the next bend (elbow). The slope should be specified by the design team. At power plants (piping in the steam/water circuit), we typically look for 1/8 inch of "drop" per foot of horizontal length.
Checking at the hangers will give us a chance to note how much hanger adjustment might be neessary to obtain the appropriate slope as we walk the system.
I am sure there are as many other ways to do this task as there are other engineers doing it.
Best Regards, John.